In an atomic absorption spectrophotometer, a background correction is normally made to avoid the influence of absorption other than that due to an object atomic element. JIS (Japanese Industrial Standards) K0121 “General rules for atomic absorption spectrochemical analysis” and EPA (Environmental Protection Agency) stipulates to use the SR method or the deuterium lamp method. The former method has an advantage over the latter in that it yields better correction accuracy and can be used in a wider range of wavelengths.
In the SR method, a self reverse type hollow cathode lamp (HCL) is used, where a smaller current of about 10 mA and a larger current of about 500 mA are alternately supplied to the HCL, as shown in FIG. 2. When the larger current is supplied, the shape of the emission spectrum of the HCL has a dip at the center like the letter “M”, so that it undergoes little atomic absorption of the object element while the background absorption mainly occurs. When, on the other hand, the smaller current is supplied, the emission spectrum becomes a narrow peak, so that it undergoes both the atomic absorption of the object element and the background absorption. Taking the subtraction of the two absorptions, solely the atomic absorption of the object element is properly extracted while the background absorption is adequately corrected.
A measure of the accuracy of the background correction is the background correcting factor α (%), which is defined by the following formula.α={[absorption by a sample without object element after background correction]/[absorption by the background]}×100
The background correcting factor a approaches to zero as the correcting accuracy is enhanced.
In the SR method, the background correcting accuracy is impaired due to various differences in the conditions between the smaller current supplying period and the larger current supplying period. The emission noise of the HCL is an example of the cause of such differences. In order to decrease the difference (or error) in the correction, in conventional atomic absorption spectrophotometers, a sample hold circuit is provided for each of the photometric signal taken in the smaller current supplying period and the photometric signal taken in the larger current supplying period, and the RC time constants of the respective sample hold circuits are appropriately set to minimize the background correcting factor (Japanese Unexamined Patent Publication No. H05-306997). In many cases, another sample hold circuit is provided for the photometric signal taken in the bias current supplying period.
In order to appropriately set the RC time constants of the two sample hold circuits, the user has to adjust the value of the trimmer resistance or trimmer capacitor manually considering the result of the analysis. Such a manual adjustment is troublesome, and another problem is that the trimmer resistance or trimmer capacitor is an analog part whose value may change gradually. Further, using plural sample hold circuits solely for the purpose of a background correction is cost inefficient.